[JS IR] Emit original names for local vars to sourcemaps

#KT-35655 Fixed

js/js.translator/testData/lineNumbers/jsCode.kt

@@ -5,5 +5,7 @@ fun foo() {
     println("after: $x")
 }
 
+// DONT_TARGET_EXACT_BACKEND: JS_IR
+// ^There is a better stepping test
+
 // LINES(JS):      1 6 2 2 3 3 4 4 5 5
-// LINES(JS_IR): 1 1   2 2 3 3 4 4 5 5